Anti-skid systems for rail vehicles carry out a similar function to an anti-lock braking system (ABS) of a motor vehicle. During the braking of a rail vehicle, a sudden stationary state of the axle, which can result in a flat point in the wheel tire which worsens the true running in the case of extended braking distance, can occur in the contact face between the wheel and the rail owing to changing adhesion conditions. A significant function of anti-skid systems is to prevent the stationary state of the axle during the braking operation, for example by suitable automatic evacuation and aeration of a pneumatic brake cylinder.
For the purpose of braking, the driver of the traction vehicle applies the brake control pressure, as a result of which after a short time a corresponding brake cylinder pressure is set in the pneumatic brake cylinders at the wheel sets. As a result of the braking torque which is introduced in this way the wheel set is decelerated, resulting in slip between the wheel and rail at the wheel contact faces. The slip is defined in the case of braking as follows:s=(ν−Rω)/ν,
where s characterizes the slip,  characterizes the vehicle speed,  characterizes the axle speed and R characterizes the nominal wheel radius. The friction force which decelerates the rail vehicle is the product of the adhesion loading which is dependent on the slip and the wheel contact force. As the slip increases, the adhesion loading rises steeply and drops slowly after a maximum value has been reached. The maximum value of the adhesion depends here on various influencing factors, inter alia on the weather conditions and the state of the rail. In the case of a smooth rail or a rail which is covered with fall foliage, it is particularly low. In the case of slip-free travel there is no adhesion loading present.
In the case of a rail vehicle, the achievable braking force or traction force is therefore proportional to the available adhesion between the wheel and the rail. The maximum available adhesion with the associated wheel slip depends here on a multiplicity of parameters. According to the current prior art, as is described e.g. in DE 10 2006 057 813 A1, essentially two slip ranges can be differentiated. On the one hand adhesion conditions which require a high wheel slip, such as is usual e.g. in the case of tests with respect to the Standard UIC 541-05 of the International Railway Association (Union internationale des chemins de fer (UIC)) in the case of aqueous intermediate layers. On the other hand, a small wheel slip if the intermediate layers are composed e.g. of fall foliage or oily substances.
An anti-skid system or a traction control system is therefore intended to set the suitable wheel slip in order to maximize the adhesion. Conventional anti-skid systems which are approved according to UIC 541-05 adjust a typical wheel slip in the slip range which is permissible by the UIC541-05. This slip range is matched to the test conditions defined in the standard, with a water-soap mixture as the adhesion. In the abovementioned DE 10 2006 057 813 A1 two different adjustment ranges are differentiated empirically on the basis of travel tests and travel mode. A normal slip range (<30%) and a low slip range (<5%). Switching over between the two slip ranges occurs depending on the time of year (fall), as a result of insufficient deceleration power in the case of high wheel slip or as a result of the evaluation of wheel set decelerations.
In further anti-skid methods, for example a model-based approach is pursued in which properties of the intermediate medium present in the wheel-rail contact are identified and the optimum wheel slip which is necessary to achieve the maximum adhesion is derived from these properties. It has also been proposed to set the slip to the adhesion maximum on the basis of axle-based measurement of the adhesion-slip relationship. Another approach is based on not carrying out adaptation for the wheel slip and instead adjusting 50% of the axles to the standard slip range (according to UIC 541-05), and the remaining 50% to an alternative small slip range.
Conventional anti-skid systems according to the prior art are optimized essentially to one type of adhesion. Adaptation to different conditions does not take place. If the current adhesion requires a small or very small wheel slip, the axles operate with non-optimum wheel slip. In the case of adaptation of the slip range, vehicle-specific parameters (such as acceleration, mass, force, etc.) have to be taken into account. There is no relatively detailed explanation of the exiting of the state of low deceleration (small slip). Singular brief deviations in the wheel set acceleration can lead to misinterpretations of the suitable slip range. A multiplicity of parameters and/or characteristic diagrams with reference data are usually required to determine the relationships between the adhesion and slip which are necessary to decide the slip range. In systems in which the estimation about the suitable wheel slip of the entire system is based on the feedback about an individual axis, a precondition is that a similar type of adhesion is present at all the axles.
The adaptation-free approach which was mentioned last above constitutes a compromise. It provides the advantage that it is not necessary to know parameters relating to the rail vehicle and it supplies a defined constant wheel slip distribution. Therefore, a maladaptation can be avoided. However, as a result, only 50% of the axles operate in the suitable slip range. The available system braking force is therefore larger than that of a mal-adapted system but smaller than that of a correctly adapted system.